Natural killer (NK) cells were implicated recently in three important biological functions: 1) in immunosurveillance to malignancies; 2) in resistance to already established malignancies; 3) in rejection of allogeneic bone marrow transplants. Because of possible involvement of NK cells in these important biological functions we plan to: a) develop new techniques for NK cell enrichment or purification; b) to prepare antisera against NK cells, so their functions, heterogeneity, tissue distribution and redistribution under normal and pathological conditions can be evaluated; c) to study involvement of NK cells in resistance to leukemia and to determine the cause underlying the NK cell defect in leukemia patients; d) to determine whether NK cells can be used as a diagnostic and prognostic factor in leukemia; e) to assay the effect of various immunomodulating agents on NK cell anti-tumor activities; f) to study the involvement of NK cells in allogeneic bone marrow graft rejection and graft-versus-host reaction and possibly to develop a new matching technique for bone marrow transplantation on the basis of NK cell activities.